This invention relates to video signal reproducing apparatus and, more particularly, to a method and apparatus for improving the sharpness of the video picture displayed on the screen of a cathode ray tube included in such video signal reproducing apparatus.
The problem of improving the sharpness of an image displayed on the screen of video display apparatus long has been a problem in the video signal reproducing art. This problem is found in both monochrome (black-and-white) and color video display apparatus. The brightness of a displayed image is, of course, dependent upon the intensity of the light emitted to form that image. In a typical display screen of a cathode ray tube (CRT) of the type conventionally used in video signal reproducing apparatus, such as in a television receiver, phosphor elements, such as dots, are provided on the screen and are adapted to be excited by an impinging electron beam, whereby a corresponding amount of light is emitted thereby in accordance with the intensity of the exciting beam. Thus, increased brightness is achieved by increasing the intensity of the electron beam which impinges upon the phosphor screen. However, when the intensity of an electron beam is increased, the size of the landing beam spot which impinges upon the screen likewise is increased. This means that the area on the screen which is excited to emit brighter light is relatively large.
The sharpness of an image displayed on the phosphor screen of a CRT is, to a great extent, determined by the size of the landing beam spot. As mentioned above, when the intensity of the electron beam is increased so as to increase the brightness of the displayed image, the size of the landing beam spot likewise is increased. Consequently, at boundaries between relatively high and low brightness levels, the demarcation between such levels is not precisely defined and, therefore, the bright image appears to be blurred or "fuzzy".
The problem of lack of sharpness in the displayed image of a CRT also is due, at least in part, to the relatively low frequency response of the video signal reproducing apparatus. For example, if a horizontal line interval of a video signal includes an abrupt change in the brightness level, the slow response of the apparatus effectively prevents the intensity of the scanning electron beam from changing in a corresponding abrupt manner. Thus, when the displayed video picture is viewed in the line-scanning direction, this relatively slow change in the intensity level of the electron beam is seen as an ill-defined demarcation or boundary. That is, the sharpness of the reproduced image is degraded at portions of the image where abrupt changes in brightness occur in response to transitions in the brightness of the video signal which is reproduced.
The effects caused by the relatively large landing beam spot for bright levels in the video picture and the relatively slow frequency response of the video signal reproducing apparatus are cumulative, resulting in an image whose sharpness is less than satisfactory. Various proposals have been addressed to this problem of image-sharpness in a video picture. In the so-called aperture compensation technique, the intensity of the electron beam first is decreased and then is increased at positive transitions in video brightness levels, and an inverse operation is performed at negative transitions in the video brightness level. However, by increasing the intensity of the electron beam to be greater than the highest level actually represented by the video signal, the resultant landing beam spot is made larger than it otherwise would be. Another proposal for improving the sharpness of a displayed image is the so-called beam-scanning velocity modulation technique. In this technique, the scanning velocity of the electron beam in the line-scanning direction is changed, or modulated, in accordance with the transitions in the brightness level of the video signal from which the displayed video picture is derived. The brightness-level transitions are used to effect a supplemental horizontal deflection of the electron beam in addition to the main horizontal deflection thereof. In particular, at the transition in the video signal from a lower to a higher brightness level, the scanning of the beam first is accelerated and then is decelerated in the line-scanning direction; and at transitions in the video signal from higher to lower brightness levels, the scanning of the beam first is decelerated and then is accelerated in the line-scanning direction. This has the effect of improving the sharpness of the displayed image in the line-scanning, or horizontal, direction. However, this beam-scanning modulation technique has no effect upon the actual sharpness of the displayed image in the vertical direction, nor does a viewer perceive any subjective, or psychological improvement in the vertically-directed sharpness of the image.